RAS "G/MIST" discussion meeting
10 March 2000

[Report published in Astronomy & Geophysics 41, 3.32-3.33 (June 2000)]

A. Programme B. Report C. Abstracts

Programme:

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Published in Astronomy & Geophysics 41, 3.32-3.33 (June 2000)

A discussion meeting on Terrestrial, Planetary and Inter-planetary magnetic fields was held at Burlington House on 10 March 2000. There were eight invited speakers, including two from overseas, and an audience of about 50 participated in the discussions after each talk.

Michelle Dougherty (Imperial College) began the proceedings by describing the magnetic fields near the four Galilean moons of Jupiter, obtained during close flybys of the Galileo orbiter. The four moons orbit in four different plasma regimes in Jupiter's magnetosphere, but it is possible from the data to infer the nature of their magnetic fields, and in particular whether they are intrinsic internal fields or induced fields. This enables inferences to be drawn about the satellites' internal structure. Ganymede, the largest satellite, has its own internal dipole field and magnetosphere. Io is more complex, but probably also has an internal field, though the data are ambiguous. Europa appears to have an induced field and possibly a subsurface ocean; its dipole was observed to have "flipped" between one flyby and the next. Callisto's induced field on the other hand is consistent with a rocky core. Similar studies of Saturn's satellites will be possible from the Cassini-Huygens mission which will reach the planet in 2002.

Peter Stauning (Danish Meteorological Institute, Denmark) gave a progress report on the Oersted mission. The first Danish satellite was launched on 23 February 1999, five years after the original planned launch date, and following 10 scrubbed countdowns. Its essential mission was to survey the terrestrial magnetic field along its orbit (near-polar 649-865 km), and for this purpose it carried vector and scalar magnetometers. It also carried a star imager for the attitude control needed for the vector measurements, and an energetic particle instrument. There were a number of technical problems, for example with the attitude control, timing, calibration, and boom deployment. These problems were nearly all solved during an extended commissioning period and since August 1999 Oersted has been producing a wealth of high quality scalar and vector geomagnetic data. So far this has contributed to the IGRF2000 model, and has also been used to model flows at a depth of 3000 km, from the geomagnetic fields at the core-mantle boundary.

Alan Thomson (British Geological Survey) dealt with the use of satellite magnetic data for modelling the Earth's core-generated field, and techniques for separating out this component from those due to other sources, e.g. crustal anomalies, or ionospheric/ magnetospheric currents. The last satellite specifically designed for this purpose had been MagSat which flew in 1979-1980, and he spent some time reviewing what had been learned from that mission before moving on to discuss Oersted with its superior instrumentation. He described the analysis he had done on the data, such as careful selection of the 2% data points used, to minimise the effect of errors and to obtain the best estimates of the core field and its rate of change.

The heliospheric magnetic field as observed by the Ulysses out-of-ecliptic mission was the subject of the talk by Bob Forsyth (Imperial College). The field is a consequence of the solar coronal magnetic field which becomes "frozen in" as the solar wind expands away from the Sun. There are two types of solar wind flow: at low heliospheric latitudes there are slow speed flows whilst at higher latitudes, Ulysses has observed high speed flows which are assumed to originate in the polar regions of the corona. Sometimes a region of high speed flow will overtake a slower one with the result that a co-rotating interaction region is formed.Ulysses is now starting on its second orbit, at approximately the opposite phase of the solar cycle to the first orbit. The observations are significantly different, with the slow speed flows being seen to a higher latitude than on the first orbit.

The next talk was from the second international speaker, Mario Acuna (NASA Goddard) who presented recent magnetic field results from NASA missions to the moon and Mars. Mars Global Surveyor flies under the Martian ionosphere in an orbit which is ideally suited to surveying the Martian crustal magnetic field. Regions of high magnetisation form ``mini-magnetospheres''. Mars no longer has a dynamo field but is believed to have done so in the past. Magnetic anomalies are observed to be largely confined to the southern, cratered, older region of the planet, with none over volcanoes; this result has implications for tectonic models of Mars' evolution. At the Moon, Lunar Prospector has found that the highest crustal fields are antipodal to the major impact basins. This could be explained by magnetic reconnection allowing magnetic flux of the intense field generated momentarily during the explosion caused by an impact to be transported around the surface to the antipodes.

After a break for lunch, the meeting resumed with a contribution from Raymond Hide (Oxford University) on a theory of geomagnetic field reversals. He began by reviewing the theory in which the Earth's magnetic field is modelled as originating in a self-exciting dynamo in the Earth's molten core, analogous to the machine patented by Varley in 1866. A dynamo mechanism for the origin of the geomagnetic field is the only one capable of generating fields as strong as those which are observed. A new feature of the theory, namely nonlinear quenching of fluctuations, together with changing conditions at the core-mantle boundary, was invoked to explain irregularities in the time series of geomagnetic polarity reversals, particularly `superchrons' --- long intervals in which reversals were absent.

Andy Jackson (Leeds University) then spoke of the use of historical magnetic field data in reconstructing the time dependence of the geomagnetic field in recent centuries. He had used observations of magnetic declination (commonly termed magnetic variation) taken on board naval and commercial ships at sea since 1590. Measurement of magnetic variation had been important for ships then, since the only method of determining longitude was by dead reckoning using a magnetic compass. The measurement was made by comparing the compass direction with the bearing of the Sun at noon, sunrise, or sunset, and was usually entered in the ship's log, often to a surprisingly good precision, typically 0.5°. The archives of trading companies such as the East India Company and the Hudson Bay Company had been trawled for data, which had then been used to produce a time-dependent global spherical harmonic field model.

The final item on the programme was a presentation by Mervyn Freeman (British Antarctic Survey) on interplanetary magnetic field fluctuations and their coupling to the geomagnetic field. He reported a scale-free structure in such fluctuations, as evidenced by a power law variation of burst sizes, durations and inter-burst intervals of the solar wind Poynting flux. Two explanations were possible, either turbulence (for example the shell model) or self-organised criticality (SOC), both of which exhibit power law burst distributions. The evidence was presently pointing towards the former. A further unresolved question was whether the scale-free behaviour exhibited by the magnetosphere (in addition to the 90-minute substorm scale) was an internal property or driven by the solar wind.

The meeting concluded with thanks to the speakers and tea at Savile Row.